W. E. Burr
25 MAR 1988
INTERFACE JARGON
Author's Disclaimer
I do not claim that what follows are carefully researched,
authoritative or rigorous definitions of the listed terms, nor
that they follow accepted vocabularies, such as those of ANSI or
ISO. In fact, surprisingly few of the terms used here are found
in those vocabularies. I have tried to follow common usage in the
X3T9 committees as I understand it, which may differ from use
elsewhere, as well as usage in the general technical literature.
Some terms which I have researched in some detail, for example ,
"bus," are used in the literature in such diverse senses that a
single, concise, all-encompassing definition, would be difficult,
if not impossible. I have also departed a bit from simple
definitions in many cases, to say a bit about why we are
interested in the subject, or to highlight the tradeoffs inherent
in the use of some things for which definitions are offered. The
purpose here is to aid the uninitiated in understanding X3T9
documents and discussions rather than to provide rigorous,
accepted definitions.
Backplane bus - A bus connecting main memory, CPU's, other
processors, peripheral device controllers and adapters to other
buses along the "backplane" of a card cage or frame. Backplane
buses make provision for addressing each memory word with each
transfer cycle. Backplane bus standards also usually include
mechanical printed circuit card standards as well. X3T9 does not
do backplane bus standards; in the US these are usually done by
the IEEE Computer Society.
Balanced signal - Signals are balanced when they consist of equal
currents moving in opposite directions. Not all differential
signals are truly balanced; small bias currents are sometimes
introduced by terminators to "pull" undriven differential signals
to a definite state, and these currents will slightly unbalance a
differential signal. When balanced or nearly balanced signals
pass through twisted pair lines, the electromagnetic effects (i.
e. RFI, crosstalk, etc.) of the two opposite currents largely
cancel each other out.
Broadcast network - A network where a station transmits and every
other station in the network receives that transmission after
some network propagation delay, without any intermediate
switching decisions. Sometimes, confusingly, called a "bus
network."
Bus - A linear conductive signal path capable of connection to
three or more attachments. In general in X3T9 it is considered
to be distinct from radial or point-to-point signal connections.
[In the general literature bus is used in a bewildering variety
of ways. It comes ultimately from the latin, "omnibus," meaning
"for all." Its usage in electrical engineering is comparatively
ancient. In power distribution a bus is a common attachment,
often a substantial copper bar, to which branch circuits are
tied. The notion of linearity is inherent in the bus bar, and in
the "bus line" of railroad trains. When used to describe a
topology, bus always implies a linear structure. Some authors
apparently use bus to describe a parallel collection of related
wires or signals, even when the connection is point-to-point.
Other authors sometimes use bus as a synonym for interface. In
networking literature (particulary for military applications) the
level of abstraction sometimes rises far above that of electrons
flowing in copper or photons in glass and we sometimes see the
notion of an information bus, which may have nothing to do with a
physically conductive path. I have seen the term "ring bus," a
simple oxymoron by my understanding of bus, apparently used to
characterize the ring network as the common information exchange
facility. Some authors use "bus network" to describe what I call
a "broadcast network," even where there is no topological
requirement for linearity and the network is star-wired.
Byte - A unit of information containing precisely 8 bits, no more
and no less. There were some ancient computers which used the
term byte for some other quantity of information and pedants
delight in telling us that byte is therefore ambiguous. The
curious term "octet" has been invented to end this apparent
ambiguity in standards, but the use of byte to mean a quantity of
8-bits is so pervasive and ubiquitous, that there is no need for
octet (which may cause the reader to wonder if the octet contains
an oboe or a bassoon). Surely there does not remain alive today
a single practitioner of computing who does not understand that
byte means 8 bits in common usage.
Cartridge Tape - When applied to magnetic tape media, this
usually refers to a 1/4 in cartridge which uses an internal belt
to drive the tape motion.
Cassette Tape - This term when applied to magnetic tape media,
usually refers to the familiar "Philips" cassette, which is
ubiquitous in audio applications and is used in some inexpensive
digital tape drives. Unlike tape cartridges (q.v.), the tape is
driven through the hub of the reels.
CD ROM - Compact Disk Read Only Memory. A read-only optical disk
storage device based upon the common audio compact disk. This
medium has extraordinary promise for electronic publishing, if
suitable data format standards can be agreed to, because it
allows reproduction of huge amounts of data by a simple stamping
process, and the basic mechanisms are in high volume production
for home entertainment applications.
Common-mode noise - Noise introduced by currents flowing through
signal ground paths. Common mode noise is frequently introduced
to signals between separate computer equipments through the power
distribution circuits. It is a particular problem when
single-ended signals are used to connect equipments which are
powered by different circuits.
Computer - A central processor unit (CPU) plus its main memory.
Control unit - A unit which controls a peripheral device and
presents a peripheral interface with a relatively high level
interface to a storage device. Typical control unit functions
may include the formatter, error detection and correction and
data encoding and decoding. This term is roughly synonymous with
controller.
Controller - See control unit.
CPU - Central processor unit. The logical engine of the computer
which interprets instructions and performs computation.
Cross talk - The electromagnetic coupling of a signal on one line
with another nearby signal line.
Data encoder - When writing to a storage device or when
transmitting on a bit serial transmission medium, the data
encoder combines separate clock and data signals to generate a
"self clocking code" which contains both the data and clock
signals.
Data separator - On magnetic media data and clock signals are
stored together in the form of flux reversals on a magnetic
surface. When reading, the data separator breaks the recorded
pattern of flux reversals into separate data and clock signals.
The data separator may be in either the device or the control
unit. There is an analogous function when receiving a serial bit
stream from a network, which also is in the form of a self-
clocking code. This is sometimes called simply a "decoder."
D/CAS - Digital Cassette. See Cassette tape.
Decoder - See data separator.
Device interface - A relatively low level interface, peculiar to
a particular type of device, between a control unit and a device.
As logic becomes cheaper more and more controller function is
being built into devices, and "intelligent" device interfaces
become difficult to distinguish from peripheral interfaces.
Device - A single storage or output unit such as a disk drive,
tape drive or printer.
Differential signal - An electrical signal where a pair of lines
are used in "push-pull" fashion. Each driver has two outputs and
each receiver has two inputs. In many cases differential signals
are "balanced," that is the same current flows on each line in
opposite directions. The advantages of differential signals (as
compared to single-ended signals) are comparatively large
tolerance for common-mode noise, and little cross-talk when used
with twisted pair wires in long cables. Differential signals
are inherently more expensive than single-ended signals, because
two pins are required for each signal on each driver and
receiver chip. This costs extra pins, chips, board real-estate
and, ultimately, money.
Dot-OR signal - See Wire-OR signal.
Formatter - Data are ordinarily recorded in storage devices in a
predetermined format typically including gaps, synchronization
fields, starting delimiters, ID fields (often including flags for
bad recording areas), the data to be stored and error detecting
or forward error correcting codes. The formatter adds these
needed fields when writing data and strips them when reading.
The formatter traditionally has been a control unit function in
older storage subsystems; newer "intelligent" devices may have
the formatter built in.
Intelligent peripheral - In the past, the tendency was to remove
most logic from individual devices and consolidate it in a
controller serving several devices. With the advent of LSI chips
it is now practical to move some or all logic formerly in
controllers into devices. Very high recording densities require
that the data separator be in the drive. At a minimum,
intelligent storage peripherals have the data separator and
formatter in the device, and may have buffering and error
detection/correction logic as well as high level command sets
implemented in the device. In this case the controller entirely
disappears into the device.
I/O interface - An interface which connects the computer to
peripheral subsystems. In some computer systems the I/O inter-
face is also a backplane bus (e. g. the UNIBUS of a PDP-11) which
also connects the processor to its main memory. In other systems
it is a separate bus with a separate memory port (e.g. the SCSI
bus). For the purposes of this discussion backplane buses,
peripheral buses, and system buses may all function as general
purpose I/O interfaces, and device interfaces as special purpose
I/O interfaces.
Motherboard bus - Similar to a backplane bus (q.v.) except that
rather than running along a card frame, it runs along a large
"mother" printed circuit card and connects the mother card to
smaller "expansion" cards. This kind of bus is used in the IBM
PC, its clones and some other personal computers.
Peripheral bus - a bus which connects a single host/computer to
several peripherals subsystems and is broadly applicable to a
range of peripheral types. Unlike a backplane bus, a peripheral
bus does not carry memory addresses. Unlike a system bus, a
peripheral bus does not facilitate host-to-host communications.
RFI - Radio Frequency Interference. Any high frequency signal
will radiate if current carrying paths are not shielded,
particularly when signal path lengths are comparable to or longer
than signal wavelengths. The FCC now regulates RFI in computer
equipment sold in the US (FCC Regulations Part 15, Subpart J).
Run length limited (RLL) - A code which guarantees that there is
some maximum period between signal transitions whatever the data.
In this sense, RLL is roughly synonymous with self clocking.
Nearly all serial recording is done using some form of RLL code,
however the term is usually reserved for those more sophisticated
group codes which allow comparatively long maximum runs between
transitions, but also guarantee some minimum run length of at
least two code bit periods between transitions, allowing higher
storage densities.
Single-ended signal - An electrical signal where a single line is
used referenced to a ground path common to other signals. In
single-ended buses intended for moderately long distances there
is commonly one ground line between each pair of signal lines
which provides some resistance to signal cross-talk. The
advantage to single-ended signals is that only one driver or
receiver pin is required per signal, plus one ground pin per IC.
As compared to differential signals this saves pins, chips,
board area and, ultimately, money. The main disadvantage to
single-ended signals is that they are vulnerable to common mode
noise.
Start/stop tape drive - A tape drive where the performance
penalty to start or stop tape motion is comparatively small.
Streaming tape drive - A tape drive which ordinarily reads or
writes records without stoping the tape motion between records.
there is always a significant performance penalty whenever tape
motion stops. Streaming tape drives are generally less expensive
than start/stop drives, but require careful programming for high
performance. They are primarily used as fixed disk backup
devices.
Subsystem - The combination of a control unit plus one or more
attached devices. Synonymous with "unit" in IPI terminology.
System interface - An interface, often a bus, which connects
computers and peripherals and allows direct computer-to-computer,
computer-to-peripheral and peripheral-to-peripheral transfers.
Unlike a backplane bus a system interface doesn't carry
word-by-word or byte-by-byte storage addresses, and is usually
inefficient for short block transfers. Unlike a peripheral bus,
any port can talk directly to any other port, thus facilitating
computer to computer communications.
Unit - This word is used with rather different meanings in
different interface standards. In SCSI a "logical unit" is an
addressable bus device. In I/O Channel compatible systems it
generally means a single box, which may contain several distinct
addressable devices.
Whitney technology - This term refers to a magnetic disk with
particulate media, thin film read/write heads and a slider which
permits lower "flying heights," and thus higher bit densities,
than the older "Winchester technology." This technology was
introduced with the IBM 3370 disk drive circa 1979. Similar
densities have since been achieved with metal film media and
ferrite heads or even ferrite heads and particulate media.
Winchester technology - The term "winchester" is frequently
loosely applied to mean any disk with a fixed or non-removable
recording medium. More precisely, the term applies to a ferrite
read/write head and slider technology with particulate media,
first employed in the IBM 3340 disk drive, circa 1973. Similar
technology has been adapted to some small removable medium
drives, which may be called "removable winchesters."